From U.S. Pat. No. 4,539,194 it is known a method for production of pure silicon where one or more calcium compounds are added to molten metallurgical grade silicon in an amount sufficient to provide molten silicon containing about 1.0 to about 10.0% by weight calcium. The calcium alloyed silicon is cast and the solidified silicon is pre-crushed and than subjected to a leaching step by using an aqueous solution of FeCl3 and HCl. This first leaching step causes disintegration at the silicon and where the resulting silicon grain after washing is subjected to a second leaching step with an aqueous solution of HF and HNo3. When the molten silicon alloyed with calcium is solidified the main part of the calcium solidifies as a calcium-silicide phase along the grain boundaries of the silicon. This calcium-silicide phase also contains a majority of other impurity elements contained in the metallurgical grade silicon, particularly iron, aluminium, titanium, vanadium, chromium and others. The calcium-silicide phase containing these impurities dissolves during the leaching steps and the impurity elements contained in the calcium-silicide phase are thus removed from the silicon particles. Very good results are obtained by the method of U.S. Pat. No. 4,539,194. It has, however, been found that not all calcium-silicide phase appears on the grain boundaries of the solidified silicon. Some of the calcium-silicide phase is isolated within the grains of silicon and in narrow channels and are consequently not available to the acid solutions during the leaching steps of U.S. Pat. No. 4,539,194.
There is thus a need for a method to further refine the silicon purified by the method of U.S. Pat. No. 4,539,194.